Catalytic reduction of organic substrates remains a key enabling industrial process that sustains several major chemical industries. There is a wide range of commercially-important reductive transformations catalyzed by transition metals catalysts. For example, the transition-metal catalyzed cleavage of polar bonds such as C—S and C—N bonds, and hydrogenation of unsaturated functional groups such as alkenes (to alkanes) are reductive transformations pertinent to fine chemicals synthesis and to the production of environmentally safe fuel from crude petroleum feedstock.
Current industrial processes employing catalytic reduction are commonly mediated by relatively expensive, rare and in some cases, toxic second- and third-row transition metals. The use of these rare transition metals raises barriers to the sustainability of these industrial processes. As an example, current technologies for the upgrading of petroleum feedstocks which include hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) are energy intensive. This is due in part to the reaction conditions required for the metal catalysts currently used for these processes. Molybdenum and tungsten catalysts, promoted by cobalt and nickel ions such as CoMoS2 and NiWS2, generally function under high temperature (e.g. about 300 to 650° C.) and high pressure (e.g. about 90 to 120 atm). These conditions contribute to refining costs of petroleum and crude oil, hence, there is a demand for cost-effective and environmentally benign catalytic processes for industrial scale production of commodity chemicals and fuels.
First row transition metals are relatively inexpensive and abundant. This makes them attractive candidates [inter alia] for catalytic hydrogenation and the processing of petroleum feedstocks. Generally, however, first-row transition metal catalysts are believed to possess intrinsically low activity.
Discrete ligand-supported metal clusters have been used to model the active sites of heterogeneous catalysts and, in some cases, catalyze organic reactions. Polymetallic catalysts typically display reactivity different from monomeric catalysts. Ligand-supported metal clusters can be classified according to the relative saturation of the metal centers: coordinatively saturated clusters are relatively stable and inert, normally requiring activation prior to use. Generally, coordinatively unsaturated clusters are thermodynamically less stabilized and more reactive. Therefore, coordinatively unsaturated clusters are good candidates for catalysis.